FIG. 1 shows the circuit diagram of a typical switching power supply according to the prior art. As shown in FIG. 1, a switching power supply 100 includes a bridge rectifier 110, a transformer 111, a secondary rectifier 112, an output filter 113, a feedback control unit 114, a photo coupler 115, a pulse-width modulator (PWM) 116, and a switching device 119. The bridge rectifier 110 is configured to rectify an input AC voltage Vin into a full-wave rectified DC voltage which is then provided to the primary winding of the transformer 111. The switching device 119 is usually implemented by a MOSFET device and the switching device 119 is connected in series with the primary winding of the transformer 111. When the switching device 119 is turned on, the primary winding of the transformer 111 receives an input current from an output terminal of the bridge rectifier 110, thereby storing energy therein. When the switching device 119 is turned off, the energy stored in the primary winding of the transformer 111 is transferred to the secondary side of the transformer 111, thereby inducing an AC voltage across the secondary winding of the transformer 111. The AC voltage induced across the secondary winding of the transformer 111 is rectified by the secondary rectifier 112 which is typically implemented by a diode rectifier into a desirable DC voltage. The DC voltage outputted from the secondary rectifier 112 is smoothed by the output filter 113 which is implemented by a filtering capacitor into an output DC voltage Vo. The output DC voltage Vo is provided to power a load 121. The feedback control unit 114 is connected to a positive terminal of the output voltage Vo and includes a voltage divider made up of at least two resistive elements. The feedback control unit 114 is configured to generate a feedback signal VFB indicative of the output voltage Vo. The feedback signal VFB is transmitted to a feedback signal input terminal FB of the pulse-width modulator 116 through the photo coupler 115 having a photo-transistor 122, and thereby allowing the pulse-width modulator 116 to maintain the output voltage Vo at a predetermined level. In particular, the pulse-width modulator 116 is configured to generate pulse signals to drive the switching device 119 to turn on and off according to a specified duty ratio.
In normal operation, the load 121 is configured to draw current from the output terminal of the switching power supply 100 to sustain its operation. In most cases, the output voltage Vo of the switching power supply 100 is always constant, and the output power of the switching power supply 100 is determined by the current drawn to the load 121. When the load 121 demands an output power greater than the switching power supply 100 can provide, the switching power supply 100 will enter into an overload state. When the switching power supply 100 enters into an overload state due to the overload of the load 121, the output voltage Vo will decrease and a large current will flow in the switching power supply 100. This would damage the circuit elements of the switching power supply 100, including the switching device 119, the secondary rectifier 112 and the load 121 due to the overheating effect.
In order to overcome the overload or load short problems, conventional pulse-width modulators have incorporated an over-load protection mechanism to protect the switching power supply from damage due to overload problems. The overload protection mechanism built in a conventional pulse-width modulator is typically configured to monitor the feedback signal inputted through the feedback signal input pin and compare the feedback signal with a specified threshold value. If the feedback signal exceeds the threshold value, an overload condition is assumed to occur. Under this condition, a switching stop signal is issued to stop the operation of the pulse-width modulator, and thereby shutting down the switching power supply.
However, a problem with this type of overload protection mechanism is that it can be triggered on load transients. When load transients occur, the output voltage of the switching power supply will undergo fluctuation and the feedback signal will have momentary rises. Thus, even if the power required by the load does not exceed the maximum output power of which the switching power supply can provide, the feedback signal would abruptly exceed the threshold value and falsely activate the internal overload protection mechanism of the pulse-width modulator. Hence, it is desirable to add a time delay between the rise of the feedback signal and the activation of the overload protection mechanism so that the brief rises of the feedback signal due to the load transients can be neglected by the pulse-width modulator.
Therefore, it is intended to develop an overload protection delay circuit for adding a time delay to the feedback signal of a switching power supply so as to enable the internal pulse-width modulator of the switching power supply to accurately detect the occurrence of overload problems.